tion and development of standardized protocols. The approach, based on aerial photography, provides detailed descriptions of the composition and distribution of riparian plant communities, breaking down riparian areas by plant type (useful for detailed modeling of wildlife).

Some weaknesses of the method are that it is limited in spatial extent because of the time and resources required for the fine-scale mapping. Because it uses aerial photography, it is only applicable to the western two-thirds of the country where vegetative differences between riparian areas and uplands are obvious. The extensive effort required for this type of assessment prevents its use as a tool for monitoring broad trends in riparian conditions across large regions over long periods of time. Although some groups are more comfortable with the finer spatial resolution and additional information provided by aerial photography (or related techniques such as airborne videography and digital imaging), the expense and analysis time required to process these forms of land cover data make them computationally and financially unsuitable for assessment of the entire United States on a frequently recurring basis. These finer resolution sources could be used by states and local resource management agencies to augment a broader national assessment and provide local detail on spatial extent and composition of riparian vegetation. FWS should help develop a uniform national program for mapping riparian areas that relies on measurements of land cover and land use from broadly available remotely sensed data, such as satellite multispectral data. As described in Box 3-5, Illinois and Oregon have mapped land use and land cover via satellite remote sensing and are using those data in land planning and resource management.

CONCLUSIONS AND RECOMMENDATIONS

Historical and current land-use practices across the United States significantly affect the hydrologic, geomorphic, and biological structure and functioning of riparian areas. Land-use practices that directly remove native vegetation such as row-crop agriculture, grazing, timber harvesting, urban development, and mining have altered the character of riparian systems. Changes in hydrologic regimes as a result of water resources development across the nation have been particularly widespread and effective in degrading riparian areas. Other effects have been more indirect in that the management of upslope areas has brought about changes in adjacent riparian areas (e.g., accelerated erosion and pollutant transport from upslope areas following development or flow modification through tile drainage). The degraded conditions caused by some land and water uses are reversible over the short term—for example, by implementing agricultural best management practices or restricting grazing. The effects of other activities, such as large dams and levees and the extensive modification of hydrology in many agricultural areas, can only partially be ameliorated.

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